Comment 11 for bug 1902254

This mainline commit, recently backported to [B,F,G] for this bug:
    "btrfs: extent-tree: kill BUG_ON() in __btrfs_free_extent()"
introduces a compile warning.

The warning has been fixed in mainline by:
    cad69d139651 btrfs: tree-checker: fix incorrect printk format
so let's pick up that fix too.

Patch submitted: https://lists.ubuntu.com/archives/kernel-team/2020-November/114752.html